Don, et Listers,
Just to (hopefully) help clarify this polar moment idea for those (very)
few who may care, the polar moment (technically "polar moment of inertia",
or PMOI) is computed by summing each mass element multiplied by the square
of its distance from the center of mass. Your statement " we like to say
that the closer the mass to the center of rotation (i.e., a spin)" is a
little vague in this regard. In fact, in free motion an object will always
rotate around its center of mass. The fact that the PMOI varies as distance
squared means that it is much more important where you place the mass than
how much mass there is. Thus, the Laifman's comment about how obvious it is
that a Tiger has a larger PMOI than an Alpine simply because of the large
masses involved is not quite so obvious since moving these masses only
slightly toward the center of mass would nullify or even more than
compensate for the larger mass of, say the engine for example. I do,
however, believe he is right, but just want to make point.
Bob
B9472134, SHO, etc.
At 05:22 PM 8/11/98 -0700, you wrote:
>Message-ID: <35D0DFE2.6F07@earthlink.net>
>Date: Tue, 11 Aug 1998 17:20:50 -0700
>From: Don Cordier <doncordier@earthlink.net>
>Reply-To: doncordier@earthlink.net
>Organization: Aviation Operations & Airport Consultant - Glendale, CA, USA
>X-Mailer: Mozilla 3.01 (Win95; I)
>MIME-Version: 1.0
>To: Steve Laifman <Laifman@flash.net>
>CC: tigers@autox..team.net
>Subject: Poloar moment...
>Content-Type: text/plain; charset=us-ascii
>Content-Transfer-Encoding: 7bit
>
>Hi Steve,
>
>Good explanation.
>
>In airplanes, we like to say that the closer the mass to the center of
>rotation (i.e., a spin), the higher the rotation speed and the more
>difficult it is to stop the rotation. This also contributes to the
>"breakaway" action of cars (just one of the factors though...).
>
>I don't know if this contributes to the discussion or causes more
>confusion?
>
>What do you think?
>
>Best regards,
>
>Don Cordier (1968 Lotus Elan S4SE Coupe S/N 36/7947)
>Aviation Consultant
>Glendale, California, USA
>doncordier@earthlink.net
>
>Office & Hangar: (818) 997-7640 (24 hours)
>FAX: (818) 553-3667
>Member: NBAA, NATA, PHPA, AOPA
>
|